Plant Sampling: Recommended Guidelines

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Sample Collection

Sampling plays a critical role in plant analysis. When analyzing the nutrient status of plants, it is essential to select the plant part for chemical analysis that reflects the status of the particular element of interest. Four samplings during a growing season are usually sufficient to characterize seasonal nutritional patterns. One sampling should be early in the growing season, two in mid-season and the last one just prior to harvest. Four samples should be collected from each field or management unit. Each sample should contain material from at least 20 plants to ensure adequate, representative material for analytical testing. Separate samples should be taken from areas that appear different from the rest of the field.

A young mature leaf is generally selected for analysis. The sample can be subdivided into blade and petiole. The status of Cl, NO3--N, NH4+-N, extractable K and P, in the form of PO4--P (2% acetic acid) are generally determined through analysis of the petiole. Blades are used when evaluating the status of K, Ca, Mg, Na, Fe, Mn, Zn, Cu, B, Mo, SO4-S and total-N in plants. For diagnostic purposes, only leaves that have recently developed symptoms should be collected for chemical analysis.

Drying Recommendations

After collection, plant material should be washed to remove any residual soil or dust. Fresh samples or those suspected to be moist should be placed into paper bags (with adequate room for air movement within the bag) and dried in a forced-air oven at 55-60°C. In general, adequate drying time is approximately 12 hours or until the material snaps or breaks easily. All samples, except freeze-drying material, should be turned every 24 hours.

Make sure that the paper bags do not have a plastic lining, which will prevent the escape of moisture and cause the plants to decompose.

Exceptions

  • Range samples take 24 hours or more to dry
  • Power plant ash samples take three days to dry
  • Solid pieces of wood take 24 hours to dry
  • Rice soils take two days to dry
  • Any material larger than gallon size will take one or more days to dry
  • Fruits and vegetables should be freeze-dried and pureed (weigh the samples before and after the freeze-drying process)
  • These time frames are based on the use of a large, forced-air oven.
  • Times need to be adjusted accordingly for other types/sizes of ovens.

Grinding Recommendations

Most plant and feed samples should be ground to pass through a 40 mesh screen. Large wood samples must be splintered into smaller pieces before grinding. Average grinding time is one minute per sample if less than 10 grams or three to four minutes for samples over 10 grams.

Exceptions

  • Grape blades-use a 20 mesh screen due to tricomb separation from leaf
  • Sugar beet petioles-use a 20 mesh screen due to high sugar content
  • Walnut leaf-use a 20 mesh screen due to fibers in veins
  • Corn stalks-use a 10 mesh screen due to high sugar content
  • Freeze-dried samples do not need to be ground as they are easily crushed manually

References

Refer to references on this page.